Laminate, semiconductor device, and method for manufacturing laminate

ABSTRACT

A mist-CVD apparatus contains a first atomizer for atomizing a first metal oxide precursor and generating a first mist of the first metal oxide precursor; a second atomizer for atomizing a second metal oxide precursor and generating a second mist of the second metal oxide precursor; a carrier-gas supplier for supplying a carrier gas to convey the first and second mists; a film-forming unit for forming a film on a substrate by subjecting the first and second mists to a thermal reaction; and a first conveyance pipe through which the first mist and the carrier gas are conveyed to the film forming chamber, a second conveyance pipe through which the second mist and the carrier gas are conveyed to the film forming chamber.

This is a Continuation of U.S. patent application Ser. No. 17/272,873filed Mar. 2, 2021, which is a 371 International Application of PCTApplication No. PCT/2019/035415 filed Sep. 9, 2019, which claims thebenefit of Japanese Patent Application No. 2018-182714, filed Sep. 27,2018. The disclosure of the prior applications is hereby incorporated byreference herein in its entirety.

TECHNICAL FIELD

The present invention relates to a laminate having a corundum-structuredcrystal layer, a semiconductor device using the laminate, and a methodfor manufacturing a laminate having a corundum-structured crystal layer.

BACKGROUND ART

As next-generation switching devices which can achieve high breakdownvoltage, low loss, and high heat resistance, attention has been focusedon semiconductor devices using gallium oxide (α-Ga₂O₃) having largebandgap. Their applications to power semiconductor devices, such asinverter, and light-emitting or -receiving devices are expected.

Mist Chemical Vapor Deposition (Mist CVD. Hereinafter, this method mayalso be referred to as “mist CVD method”) has been developed by whichcrystal is grown on a substrate using a raw material atomized into amist form. This method enables production of gallium oxide (α-Ga₂O₃)having a corundum structure (Patent Document 1). In this method, agallium compound such as gallium acetylacetonate is dissolved in an acidsuch as hydrochloric acid to prepare a precursor. This precursor isatomized to generate raw-material fine particles. A gas mixture in whichthe raw-material fine particles are mixed with a carrier gas is suppliedto a surface of a substrate having corundum structure, such as sapphire.The raw-material mist is allowed to react, so that a single-orientationgallium oxide thin film is epitaxially grown on the substrate.

Such hetero-epitaxial growth has problems, particularly lattice mismatchbetween materials of a substrate and a thin film, or crystal defectattributable to substrate quality. ELO (Epitaxial Lateral Overgrowth)has been known as a method for suppressing these crystal defects. Inthis technique, for example, an amorphous thin film is used to form amask on a substrate surface so as to attain selective epitaxial growthfrom a partially exposed surface of the substrate and lateral growth onthe mask. Hence, dislocation is stopped by the mask or is curveddepending on the crystal orientation, so that defects in the epitaxialfilm are reduced. Patent Document 2 discloses an example in which anα-Ga₂O₃ thin film is formed using SiO₂ as a mask.

CITATION LIST Patent Literature

-   Patent Document 1: JP 5793732 B-   Patent Document 2: JP 2016-100592 A

SUMMARY OF INVENTION Technical Problem

High-quality corundum-structured crystals with sufficiently suppressedcrystal defects are not obtained by the mist CVD method disclosed inPatent Document 1 mentioned above.

Meanwhile, as the mask for selective growth used in the ELO methoddescribed above in Patent Document 2, an amorphous film which isdifferent in composition from the epitaxial film is employed. For thisreason, such a mask is normally formed using a film forming apparatuswhich is different from an apparatus for forming a desired epitaxialfilm. Meanwhile, the mask is patterned generally by photolithography.Thus, if the epitaxial growth by the conventional ELO method is adoptedto obtain a high-quality crystal thin film with suppressed crystaldefect, the complicated process brings about problems of increasedthroughput and production cost.

The present invention has been made to solve the above-describedproblems. An object of the present invention is to provide a laminatehaving high-quality corundum-structured crystal with sufficientlysuppressed crystal defects, and to provide a crystal production methodthat makes it possible to obtain a high-quality crystal thin film havingfew crystal defects at low cost.

Solution to Problem

The present invention has been made to achieve the object, and providesa laminate comprising:

a crystal substrate;

a middle layer formed on a main surface of the crystal substrate, themiddle layer comprising a mixture of an amorphous region in an amorphousphase and a crystal region in a crystal phase having a corundumstructure mainly made of a first metal oxide; and a crystal layer formedon the middle layer and having a corundum structure mainly made of asecond metal oxide.

Such a laminate has a low-defect, high-quality corundum-structuredcrystal layer because the amorphous region in the middle layer stopsextension of dislocation defect attributable to the substrate.

In this laminate, the crystal region may be an epitaxially grown layerfrom a crystal plane of the crystal substrate.

Thus, the crystal region of the middle layer serves as a seed crystal,resulting in a high-quality crystal layer according to the crystalorientation of the crystal substrate.

In the laminate, the middle layer may comprise the crystal region in aproportion of 1% or more in an arbitrary cross section of the middlelayer which is cut perpendicularly to the main surface of the crystalsubstrate.

Thereby, the middle layer more effectively reduces dislocation defectextending from the crystal substrate, and the crystal layer has higherquality.

In the laminate, the middle layer may comprise the crystal region in aproportion of 4% or more and 25% or less in an arbitrary cross sectionof the middle layer which is cut perpendicularly to the main surface ofthe crystal substrate.

The resulting crystal layer has further higher quality.

In the laminate, the middle layer may have a thickness of 1 nm or more.

Thereby, the middle layer is capable of more effectively inhibitingdislocation defect extending from the crystal substrate, so that thecrystal layer has higher quality.

In the laminate, the middle layer may have a thickness of 10 nm or more.

The resulting crystal layer has further higher quality.

In the laminate, the first metal oxide may mainly comprise an oxidecontaining any of aluminum, titanium, vanadium, chromium, iron, gallium,rhodium, indium, and iridium.

Moreover, the second metal oxide in the laminate may mainly comprise anoxide containing any of aluminum, titanium, vanadium, chromium, iron,gallium, rhodium, indium, and iridium.

Thereby, the laminate has the crystal layer that is excellent inelectrical properties and more suitable for semiconductor devices.

In the laminate, the middle layer may further comprise silicon.

This enables more reliable formation of the middle layer that can formhigh-standard crystal layer.

In this laminate, the silicon concentration in the middle layer can be0.5 at % or more, more preferably 1 at % or more and 10 at % or less.

This enables further reliable formation of the middle layer including amixture of the crystal region and the amorphous region.

The laminate may further comprise a stress relief layer between thecrystal substrate and the middle layer.

This further improves the crystallinity of the crystal region in themiddle layer, and further improves the crystallinity of the crystallayer.

Herein, a semiconductor device comprising at least a semiconductor layerand an electrode can be provided, wherein

the semiconductor layer comprises at least a portion of theabove-described laminate.

Accordingly, the semiconductor device has higher performance.

The present invention can provide a method for manufacturing a laminate,comprising steps of:

forming a first mixture gas in which an atomized first metal oxideprecursor, a carrier gas, and silicon are mixed;

forming a middle layer by supplying the first mixture gas onto a heatedcrystal substrate, wherein the middle layer contains a mixture of anamorphous region in an amorphous phase and a crystal region in a crystalphase having a corundum structure mainly made of a first metal oxide;

forming a second mixture gas in which an atomized second metal oxideprecursor and a carrier gas are mixed; and

forming a crystal layer on the middle layer by supplying the secondmixture gas onto the heated crystal substrate, wherein the crystal layerhas a corundum structure mainly made of a second metal oxide.

Such a method for manufacturing a laminate does not require dedicatedmask formation, and can easily form a high-quality corundum-structuredcrystal layer at low cost.

In this method for manufacturing a laminate, in the step of forming thefirst mixture gas, the silicon may be added while the atomized firstmetal oxide precursor and the carrier gas are being conveyed toward thecrystal substrate.

This enables simple formation of the middle layer containing a mixtureof the crystal region and the amorphous region.

In this method for manufacturing a laminate, the silicon may be addedwhile the atomized first metal oxide precursor and the carrier gas areconveyed through a pipe made of a silicone resin.

This enables simpler formation of the middle layer containing a mixtureof the crystal region and the amorphous region.

In the method for manufacturing a laminate, supply amount of the firstmixture gas can be changed in the step of forming the middle layer.

This enables easy control of the crystal-region proportion in the middlelayer and the thickness of the middle layer.

Advantageous Effects of Invention

As described above, the present invention makes it possible to provide alaminate having a high-quality corundum-structured crystal layer withsuppressed crystal defect. Moreover, the present invention enablessimple and low-cost production of the laminate having a high-qualitycorundum-structured crystal layer.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a view showing an embodiment of a laminate structure accordingto the present invention.

FIG. 2 is a view showing another embodiment of the laminate structureaccording to the present invention.

FIG. 3 is a view showing an example of a Schottky barrier diode (SBD)according to the present invention.

FIG. 4 is a view showing an example of a high electron mobilitytransistor (HEMT) according to the present invention.

FIG. 5 is a view showing an example of a metal-oxide-semiconductorfield-effect transistor (MOSFET) according to the present invention.

FIG. 6 is a view showing an example of an insulated gate bipolartransistor (IGBT) according to the present invention.

FIG. 7 is a view showing an example of a light-emitting device diode(LED) according to the present invention.

FIG. 8 is a view showing an embodiment of a mist CVD apparatus used in amethod for manufacturing a laminate according to the present invention.

FIG. 9 is a view showing a TEM image of Example.

FIG. 10 is a view showing a TEM image of Comparative Example.

FIG. 11 is a graph showing silicon concentration distributions inlaminates of Example and Comparative Example.

DESCRIPTION OF EMBODIMENTS

Hereinafter, the present invention will be described in detail, but thepresent invention is not limited thereto.

As noted above, there is a demand for a high-quality corundum-structuredcrystal with sufficiently suppressed crystal defect.

The present inventor and colleagues have earnestly studied this problem,and consequently conceived a laminate including: a crystal substrate; amiddle layer formed on a main surface of the crystal substrate, themiddle layer containing a mixture of an amorphous region in an amorphousphase and a crystal region in a crystal phase having a corundumstructure mainly made of a first metal oxide; and a crystal layer formedon the middle layer and having a corundum structure mainly made of asecond metal oxide. The inventor has found that such laminate has alow-defect, high-quality corundum-structured crystal layer. Thesefindings have led to the completion of the present invention.

Moreover, there is a demand for a method for manufacturing a laminate atlow cost, the laminate having high-quality corundum-structured crystalwith few crystal defects, as noted above.

The present inventor and colleagues have earnestly studied this problem,and consequently conceived a method for manufacturing a laminate,including steps of: forming a first mixture gas in which an atomizedfirst metal oxide precursor, a carrier gas, and silicon are mixed;forming a middle layer by supplying the first mixture gas onto a heatedcrystal substrate, so that the middle layer contains a mixture of anamorphous region in an amorphous phase and a crystal region in a crystalphase having a corundum structure mainly made of a first metal oxide;forming a second mixture gas in which an atomized second metal oxideprecursor and a carrier gas are mixed; and forming a crystal layer onthe middle layer by supplying the second mixture gas onto the heatedcrystal substrate, so that the crystal layer has a corundum structuremainly made of a second metal oxide. The inventor has found thathigh-quality crystal thin films with few crystal defects can be obtainedat low cost by this method for manufacturing a laminate. These findingshave led to the completion of the present invention.

Hereinbelow, the description will be given with reference to thedrawings.

FIG. 1 shows a laminate 100 according to the present invention. Thelaminate 100 includes: a crystal substrate 101; a middle layer 102formed on a main surface of the crystal substrate 101, the middle layer102 containing a mixture of a crystal region 102 a in a crystal phasehaving a corundum structure mainly made of a first metal oxide, and anamorphous region 102 b in an amorphous phase; and a crystal layer 103formed on the middle layer 102 and having a corundum structure mainlymade of a second metal oxide. Multiple columnar crystal regions 102 aare formed randomly, and the amorphous region 102 b is formed to fill agap between the crystal regions 102 a. Moreover, the crystal layer 103is formed by epitaxial growth from the crystal regions 102 a.

The crystal substrate 101 may be a known substrate and is notparticularly limited, as long as the substrate contains crystal as themain component. The crystal substrate 101 may be any of an insulatorsubstrate, an electro-conductive substrate, and a semiconductorsubstrate. Alternatively, the crystal substrate 101 may be a singlecrystal substrate or polycrystalline substrate. In view of quality andcost, a sapphire substrate is preferably used, for example.

Examples of the usable sapphire substrate include c-plane sapphiresubstrate, m-plane sapphire substrate, a-plane sapphire substrate, etc.Further, the sapphire substrate may have an off-angle. The off-angle isnot particularly limited, but is preferably 0° to 15°. Incidentally, thethickness of the crystal substrate is not particularly limited, but ispreferably approximately 200 to 800 μm in view of handleability andcost.

Herein, the metal element constituting the first metal oxide may be thesame as or different from one constituting the second metal oxide.Additionally, the phrase “mainly made of a first metal oxide” means thata dopant, inevitable impurities, etc. may be present besides the firstmetal oxide, and indicates that the content of the first metal oxide isabout 50% or more, for example. The same applies to the second metaloxide.

In the example illustrated in FIG. 1 , the middle layer 102 is directlyformed on the crystal substrate 101, but the middle layer may be formedon another layer that is formed on the crystal substrate. Particularly,when the lattice mismatch between the crystal substrate and the crystalregion causes a problem, a stress relief layer, for example, can beprovided between the crystal substrate 101 and the middle layer 102.

FIG. 2 shows a laminate 200 provided with a stress relief layer 204.Like the laminate 100 shown in FIG. 1 , the laminate 200 includes: acrystal substrate 201; a middle layer 202 containing a mixture of acrystal region 202 a and an amorphous region 202 b in an amorphousphase; and a crystal layer 203 formed on the middle layer 202. Further,the stress relief layer 204 is provided between the crystal substrate201 and the middle layer 202. This can further improve the crystallinityof the crystal region 202 a in the middle layer 202, and consequentlycan further improve the crystallinity of the crystal layer 203.

The stress relief layer 204 is preferably formed to relieve the latticemismatch between the crystal substrate 201 and the crystal region 202 a,for example. In this case, values of the lattice constant of the stressrelief layer 204 preferably vary in a continuous or stepwise manner in agrowth direction of the stress relief layer 204 from a similar orequivalent value to that of the lattice constant of the crystalsubstrate 201 to a similar or equivalent value to that of the latticeconstant of the crystal region 202 a. For example, in a case where amiddle layer containing an α-Ga₂O₃ crystal region is formed on an Al₂O₃substrate, the stress relief layer 204 is preferably formed from(Al_(x)Ga_(1-x))₂O₃ (0≤x≤1) such that the value of x is decreased fromthe crystal substrate 201 side to the middle layer 202 side.

Hereinbelow, description will be given of common aspects between thelaminate 100 shown in FIG. 1 and the laminate 200 shown in FIG. 2 .

The amorphous regions 102 b, 202 b in the middle layers 102, 202particularly have an effect of preventing spread of dislocation defector the like attributable to the crystal substrate. The crystal regions102 a, 202 a function as seed crystals of the crystal layers 103, 203.

The processes of forming the crystal layers 103, 203 may involve notonly epitaxial growth from the crystal regions 102 a, 202 a, but alsocrystal growth from nuclei formed on surfaces of the amorphous regions102 b, 202 b in the middle layers 102, 202. Unlike the epitaxial growthfrom the crystal regions 102 a, 202 a, the crystal grown from the nucleiformed on the surfaces of the amorphous regions 102 b, 202 b has randomcrystal orientations. For this reason, it is preferable to set ratios ofthe amorphous regions 102 b, 202 b and the crystal regions 102 a, 202 awithin an appropriate range in order to form the crystal layers 103, 203with higher quality. Hence, in arbitrary cross sections cutperpendicularly to each main surface of the crystal substrates 101, 201,the middle layers 102, 202 preferably contain the crystal regions 102 a,202 a in a proportion of 1% or more, more preferably 4% or more and 25%or less. Note that this proportion is based on areas of the regions ineach cross section.

The crystal layers 103, 203 can have higher quality when the proportionsof the crystal regions 102 a, 202 a in the middle layers 102, 202 arewithin such ranges.

Moreover, the middle layers 102, 202 each have a thickness of preferably1 nm or more, more preferably 10 nm or more. When the thickness of themiddle layers 102, 202 is within such ranges, the effect of preventingspread of dislocation defect etc. attributable to the crystal substratescan be enhanced.

In the middle layers 102, 202, the metal oxide is not particularlylimited, as long as it can have a corundum structure. The metal oxidecan be, for example, mainly made of an oxide containing any of aluminum,titanium, vanadium, chromium, iron, gallium, rhodium, indium, andiridium; more specifically, Al₂O₃, Ti₂O₃, V₂O₃, Cr₂O₃, Fe₂O₃, Ga₂O₃,Rh₂O₃, In₂O₃, or Ir₂O₃. Alternatively, the metal oxide can be a binarymetal oxide shown by (A_(x)B_(1-x))₂O₃ (0<x<1), where A and B representtwo elements selected from the aforementioned metal elements, or aternary metal oxide shown by (Al_(x)B_(y)C_(1-x-y))₂O₃ (0<x<1, 0<y<1),where A, B, and C represent three elements selected from theaforementioned metal elements.

Additionally, the middle layers 102, 202 preferably contain silicon.Incorporating silicon in the middle layers 102, 202 enables morereliable formation of the middle layers 102, 202 containing mixtures ofthe crystal regions 102 a, 202 a and the amorphous regions 102 b, 202 b.

In this case, the silicon concentration can be 0.5 at % or more, morepreferably 1 at % or more and 10 at % or less. When the siliconconcentration is 0.5 at % or more, the amorphous regions 102 b, 202 bcan be more reliably formed. Meanwhile, when the silicon concentrationis 10% or less, the crystal regions 102 a, 202 a can be more reliablyformed.

Note that it is found that in the middle layers 102, 202, the oxygenproportion is higher than the stoichiometry of the metal oxide forforming the middle layers 102, 202. Based on this, presumably, siliconadded to the middle layers 102, 202 forms silicon oxide, and functionsto disturb the crystal structure on the crystal substrate surface belowthe middle layer, thereby promoting the formation of the amorphousregion.

In the crystal layers 103, 203, the metal oxide is not particularlylimited, as long as it can have a corundum structure. The metal oxidecan be, for example, mainly made of an oxide containing any of aluminum,titanium, vanadium, chromium, iron, gallium, rhodium, indium, andiridium; more specifically, Al₂O₃, Ti₂O₃, V₂O₃, Cr₂O₃, Fe₂O₃, Ga₂O₃,Rh₂O₃, In₂O₃, or Ir₂O₃. Alternatively, the metal oxide can be a binarymetal oxide shown by (A_(x)B_(1-x))₂O₃ (0<x<1), where A and B representtwo elements selected from the aforementioned metal elements, or aternary metal oxide shown by (Al_(x)B_(y)C_(1-x-y))₂O₃ (0<x<1, 0<y<1),where A, B, and C represent three elements selected from theaforementioned metal elements.

Further, the crystal layers 103, 203 each may have a monolayer structuremade of the metal oxide, or may have a stacked structure of multiplecrystal films having different compositions and components such asdopant.

When the metal oxide in the middle layers 102, 202 and/or the crystallayers 103, 203 is such metal oxide as described above, the resultingcrystal layer or laminate is more suitable for semiconductor devices.

Further, each of the crystal substrates 101, 201, the middle layers 102,202, and the crystal layers 103, 203 in the inventive laminates 100, 200may be doped with an impurity to impart electric conductivity. As suchan impurity, for example, when the metal oxide contains at leastgallium, any of silicon, germanium, tin, magnesium, and copper, or acombination thereof can be suitably used. The concentration of theimpurity added by the doping can be appropriately set depending on thetarget final product. The concentration can be, for example, 1×10¹⁶ cm⁻³or more and 8×10²² cm⁻³ or less. Moreover, the crystal layers 103, 203can be stacked layers of multiple crystal layers doped at differentimpurity concentrations.

The crystal layer in the inventive laminate has low defect density, andis excellent in electrical properties and industrially useful. Such alaminate is suitably usable for semiconductor devices and so forth,particularly useful for power devices. Moreover, the crystal layerformed as a portion of the laminate may be used as it is (i.e., in thelaminate), or may be applied to a semiconductor device or the likeafter, for example, separated from the crystal substrate and so on by aknown method.

Meanwhile, semiconductor devices can be classified into: horizontalelement (horizontal device) having an electrode formed on one side of asemiconductor layer; and vertical element (vertical device) havingelectrodes respectively on both of top and bottom sides of asemiconductor layer. At least a portion of the inventive laminate issuitably usable in both of horizontal and vertical devices. Theinventive laminate is particularly preferably used in a vertical device.

Examples of the semiconductor device include Schottky barrier diode(SBD), metal-semiconductor field-effect transistor (MESFET), highelectron mobility transistor (HEMT), metal-oxide-semiconductorfield-effect transistor (MOSFET), junction field-effect transistor(JFET), insulated gate bipolar transistor (IGBT), light-emitting diode(LED), etc.

The crystal layer obtained from the inventive laminate is applicable ton type semiconductor layers (such as n+ type semiconductor and n−semiconductor layer). Suitable examples thereof will be described usingthe drawings, but the present invention is not limited to theseexamples.

Note that semiconductor devices exemplified below may further includeother layers (e.g., insulator layer and conductor layer) and the likedepending on the specification or purpose. A middle layer, a bufferlayer, and the like may be added or omitted as appropriate.

FIG. 3 is an example of a Schottky barrier diode (SBD). An SBD 400includes: an n− type semiconductor layer 401 a doped at relatively lowconcentration; an n+ type semiconductor layer 401 b doped at relativelyhigh concentration; a Schottky electrode 402; and an ohmic electrode403.

Materials of the Schottky electrode 402 and the ohmic electrode 403 maybe known electrode materials. Examples of the electrode materialsinclude metals, such as aluminum, molybdenum, cobalt, zirconium, tin,niobium, iron, chromium, tantalum, titanium, gold, platinum, vanadium,manganese, nickel, copper, hafnium, tungsten, iridium, zinc, indium,palladium, neodymium, and silver; alloys thereof; metal oxide conductivefilms, such as tin oxide, zinc oxide, rhenium oxide, indium oxide,indium-tin oxide (ITO), and indium-zinc oxide (IZO); organicelectro-conductive compounds, such as polyaniline, polythiophene, andpolypyrrole; mixtures and laminates thereof; etc.

The Schottky electrode 402 and the ohmic electrode 403 can be formed byknown means, such as, for example, a vacuum deposition method or asputtering method. More specifically, for example, when two of theabove-described metals are used as a first metal and a second metal toform a Schottky electrode, a layer made of the first metal and a layermade of the second metal are stacked, and the layer made of the firstmetal and the layer made of the second metal are patterned by utilizingphotolithography technique, so that the Schottky electrode can beformed.

When a reverse bias is applied to the SBD 400, a depletion layer (notshown) expands in the n− type semiconductor layer 401 a, so that the SBDhas a high breakdown voltage. Meanwhile, when a forward bias is applied,electrons flow from the ohmic electrode 403 to the Schottky electrode402. Thus, the SBD of the present invention is excellent for highbreakdown voltage and large current, the switching speed is fast, andthe breakdown voltage and reliability are also excellent.

FIG. 4 is an example of a high electron mobility transistor (HEMT). AHEMT 500 includes an n type semiconductor layer 501 with wide band gap,an n type semiconductor layer 502 with narrow band gap, an n+ typesemiconductor layer 503, a semi-insulator layer 504, a buffer layer 505,a gate electrode 506, a source electrode 507, and a drain electrode 508.

FIG. 5 is an example of a metal-oxide-semiconductor field-effecttransistor (MOSFET). A MOSFET 600 includes an n− type semiconductorlayer 601, an n+ type semiconductor layers 602 and 603, a gate insulatorfilm 604, a gate electrode 605, a source electrode 606, and a drainelectrode 607.

FIG. 6 is an example of an insulated gate bipolar transistor (IGBT). AnIGBT 700 includes an n type semiconductor layer 701, an n− typesemiconductor layer 702, an n+ type semiconductor layer 703, a p typesemiconductor layer 704, a gate insulator film 705, a gate electrode706, an emitter electrode 707, and a collector electrode 708.

FIG. 7 is an example of a light-emitting diode (LED). An LED 800includes a first electrode 801, an n type semiconductor layer 802, alight-emitting layer 803, a p type semiconductor layer 804, atransparent electrode 805, and a second electrode 806.

Examples of the material of the transparent electrode 805 include oxideelectro-conductive materials containing indium or titanium, etc. Morespecific examples thereof include In₂O₃, ZnO, SnO₂, Ga₂O₃, TiO₂, andCeO₂; mixed crystals of two or more thereof; materials doped therewith;etc. When these materials are disposed by known means such assputtering, the transparent electrode 805 can be formed. Additionally,after the transparent electrode 805 is formed, thermal anneal may beperformed to make the transparent electrode 805 transparent.

Examples of the material of the first electrode 801 and the secondelectrode 806 include metals, such as aluminum, molybdenum, cobalt,zirconium, tin, niobium, iron, chromium, tantalum, titanium, gold,platinum, vanadium, manganese, nickel, copper, hafnium, tungsten,iridium, zinc, indium, palladium, neodymium, and Ag; alloys thereof;metal oxide conductive films, such as tin oxide, zinc oxide, rheniumoxide, indium oxide, indium-tin oxide (ITO), and indium-zinc oxide(IZO); organic electro-conductive compounds, such as polyaniline,polythiophene, and polypyrrole; mixtures thereof; etc. The method offorming the electrode films is not particularly limited. The electrodescan be formed by a method appropriately selected in consideration of thesuitability to the material and so forth. Examples of the methodincludes printing method; wet processes, such as spraying and coatingmethods; physical processes, such as vacuum deposition method,sputtering method, and ion plating method; chemical processes, such asCVD and plasma CVD methods; etc.

Next, an example of a method for manufacturing the inventive laminateshown in FIG. 1 will be described with reference to FIG. 8 . However,the present invention is not limited thereto.

FIG. 8 shows an example of an apparatus used in the method formanufacturing a laminate according to the present invention. In theinventive method for manufacturing a laminate, a mist CVD apparatus 300is used. First, a first metal oxide precursor 312 a and a second metaloxide precursor 312 b are accommodated as raw-material solutions inatomizers 302 a, 302 b, respectively, and atomized to form mists (alsoreferred to as “mist generation”) by using known means. Examples of thefirst metal oxide precursor 312 a and the second metal oxide precursor312 b include gallium solutions obtained by dissolving organometalliccomplexes (e.g., acetylacetonate complex etc.) of a metal or metalgallium in acid solutions; aqueous solutions of halides (e.g., fluoride,chloride, bromide, iodide, etc.); etc. The metal is not limited, as longas the metal can form a corundum structure as metal oxide crystal.Examples of the metal include aluminum, titanium, vanadium, chromium,iron, gallium, rhodium, indium, and iridium. Moreover, the metalcontained in one of the first metal oxide precursor 312 a and the secondmetal oxide precursor 312 b may be identical to or different from theother.

The metal content in each raw-material solution is not particularlylimited, and can be appropriately set depending on the purpose orspecification. The metal content is preferably 0.001 mol % to 50 mol %,more preferably 0.01 mol % to 5 mol %.

In a case where electric conductivity is imparted to at least a portionof the laminate, doping may be performed. In this case, the impuritymaterial is not particularly limited. For example, a complex or compoundeach containing silicon, germanium, tin, magnesium, or copper can besuitably used, when at least gallium is contained as the metal.Particularly, in a case where n type electric conductivity is imparted,tin halide is preferably used. Such impurity materials can be mixed andused in an amount of 0.1 to 20 at %, more preferably 1 to 10 at %, inthe raw-material solution.

The solvent of the raw-material solution is not particularly limited,may be an inorganic solvent, such as water, may be an organic solvent,such as an alcohol, or may be a mixture solvent of an inorganic solventwith an organic solvent. Water is preferably used.

Further, the mist CVD apparatus 300 includes means for supplying acarrier gas 301. The carrier gas 301 is mixed with atomized raw-materialsolutions (metal oxide precursors) formed in the atomizers 302 a, 302 b,and conveyed to a film forming chamber 309.

The example illustrated in FIG. 8 shows a structure in which theatomizer 302 b is connected to the film forming chamber 309 with aconveyance pipe 306, and a conveyance pipe (silicon supplemental pipe)303 from the atomizer 302 a merges with an intermediate portion of theconveyance pipe 306. Nevertheless, the conveyance pipe (siliconsupplemental pipe) 303 and the conveyance pipe 306 may be connected tothe film forming chamber 309 independently of each other. Incidentally,the conveyance pipe (silicon supplemental pipe) 303 will be described indetail later.

The carrier gas 301 is not particularly limited. For example, inertgases, such as nitrogen and argon, or reducing gases, such as hydrogengas and forming gas, are suitably used besides air, oxygen, and ozone.Regarding the type of the carrier gas, one type or two or more types maybe used. The flow rate of the carrier gas can be appropriately setdepending on the size of the substrate and the volume of the filmforming chamber, and can be approximately 0.01 to 40 L/minute.

In addition, although not shown, it is also possible to add a dilutiongas to adjust the ratio between the atomized raw material and thecarrier gas. The flow rate of the dilution gas can be appropriately set,and can be 0.1 to 10 times as high as that of the carrier gas perminute. The dilution gas may be supplied downstream of the atomizers 302a, 302 b, for example. The dilution gas to be used may be the same as ordifferent from the carrier gas.

The structure and so forth of the film forming chamber 309 are notparticularly limited. A metal, such as aluminum or stainless steel, maybe used. When a film is formed at a temperature higher than theheat-resistance temperatures of these metals, quartz or silicon carbidemay be used. Inside or outside the film forming chamber 309, heatingmeans 310 is provided to heat a crystal substrate 307.

Moreover, the crystal substrate 307 may be placed on a susceptor 308disposed in the film forming chamber 309.

(Step of Forming First Mixture Gas)

First, a first mixture gas 313 is formed in which a carrier gas 301, anatomized first metal oxide precursor formed in the atomizer 302 a, andsilicon are mixed. In the example illustrated in FIG. 8 , the firstmixture gas 313 is formed by adding silicon while the carrier gas 301and the atomized first metal oxide precursor are being conveyed towardthe crystal substrate 307 placed in the film forming chamber 309. As themethod of adding silicon during the conveyance, passing the carrier gas301 and the atomized first metal oxide precursor through the conveyancepipe (silicon supplemental pipe) 303 to add silicon is simple andpreferable. As the conveyance pipe (silicon supplemental pipe) 303, forexample, a silicone-resin pipe mainly made of a silicone resin isusable. For example, methyl silicone rubber, vinyl-methyl siliconerubber, phenyl-methyl silicone rubber, and the like are suitably usable.

Alternatively, instead of using the conveyance pipe (siliconsupplemental pipe) 303 to add silicon, it is also possible to adopt amethod in which a silicon raw material is added to the carrier gas 301in advance, or a silicon raw material is added into the atomizer 302 a,for example. Nevertheless, adding silicon just through the conveyancepipe (silicon supplemental pipe) 303 as described above is quite an easyway of achieving the first-mixture-gas formation.

(Step of Forming Middle Layer)

The first mixture gas 313 formed in the above-described manner isconveyed onto the crystal substrate 307 which is placed on the susceptor308 in the film forming chamber 309, and which has been heated by theheating means 310. Thus, a middle layer is formed which contains amixture of an amorphous region in an amorphous phase and a crystalregion in a crystal phase having a corundum structure mainly made of afirst metal oxide.

Since a silicon compound such as a siloxane derived from the siliconeresin is mixed in the first mixture gas 313 inside the conveyance pipe(silicon supplemental pipe) 303 in the conveyance process, this siliconpresumably forms silicon oxide inside the middle layer during the middlelayer formation, and partially disturbs the crystal structure of, forexample, gallium oxide, so that the amorphous region is partiallyformed.

The supply of the first mixture gas 313 to the film forming chamber 309is appropriately adjusted by opening and closing a valve 304, andstopped after the middle layer with a desired thickness is formed.

Here, while the middle layer is being formed, the supply amount of thefirst mixture gas 313 may be changed. This makes it possible to easilycontrol the crystal region proportion in the middle layer and thethickness of the middle layer. Hence, the productivity improvement andlow-cost production of the laminate are possible.

Note that, in FIG. 8 , the valve 304 is disposed upstream of theatomizer 302 a, but the arrangement is not limited thereto. The valve304 may be disposed downstream of the atomizer 302 a.

(Step of Forming Second Mixture Gas)

Further, a second mixture gas 323 is formed in which an atomized secondmetal precursor (mist) formed in the atomizer 302 b and a carrier gas301 are mixed. The second mixture gas 323 is formed in the same manneras in the first mixture gas, except that silicon is not added.

(Step of Forming Crystal Layer on Middle Layer)

The second mixture gas 323 formed as described above is conveyed ontothe heated crystal substrate 307 placed on the susceptor 308 in the filmforming chamber 309. Thereby, a crystal layer having a corundumstructure mainly made of a second metal oxide is formed on the middlelayer.

The second mixture gas 323 introduced in the film forming chamber 309undergoes reaction on the crystal substrate 307 heated by the heatsource 310 to thereby form the crystal layer in the film forming chamber309.

The temperature of the crystal substrate 307 is appropriately setaccording to the raw material to be used and the target product to beformed. Nevertheless, when the middle layer is formed, the temperatureis preferably 350° C. or more and 600° C. or less, more preferably 400°C. or more and 500° C. or less. Within such temperature ranges, themiddle layer including a mixture of the crystal region and the amorphousregion can be more reliably formed.

Moreover, when, for example, α-phase gallium oxide is formed as thecrystal layer on the middle layer, the temperature is preferably 380° C.or more and 900° C. or less. With such a temperature range, the crystallayer of gallium oxide not in the 13, phase but in the a phase can bemore reliably formed.

The film formation may be performed under pressure, reduced pressure, oratmospheric pressure, preferably under atmospheric pressure in view ofapparatus cost and productivity.

Note that the film thickness can be set by adjusting the film-formationtime and the carrier-gas flow rate.

Furthermore, when the middle layer is formed, the mixture gas suppliedto the film forming chamber 309 may be the first mixture gas 313 alone,or may be both of the first mixture gas 313 and the second mixture gas323. When the first mixture gas 313 and the second mixture gas 323 aresupplied simultaneously, for example, a total flow rate of the carriergases flowing to the atomizers 302 a and 302 b is preferably within theabove-described flow rate range. Note that, in the drawing, the valve304 and a valve 305 are disposed upstream of the atomizers 302 a, 302 b,but may be disposed downstream of the atomizers 302 a, 302 b.

Example

Hereinafter, the present invention will be described in detail byshowing Example. However, the present invention is not limited thereto.

Example

A laminate was manufactured using a mist CVD apparatus similar to thatin FIG. 8 .

Two sprayers (sprayer A, sprayer B) serving as the atomizers and a filmforming chamber made of quartz were prepared. The sprayer A wasconnected to the film forming chamber with a quartz pipe. The sprayer Bwas connected, through a pipe made of a silicone resin, to the quartzpipe connected to the sprayer A before the film forming chamber.

Next, an aqueous solution containing 0.02 mol/L of galliumacetylacetonate was mixed with hydrochloric acid with a concentration of34% such that the volume ratio of the latter was 1%. The mixture wasstirred with a stirrer for 60 minutes to obtain a precursor. With thisprecursor, the two sprayers (sprayer A, sprayer B) were filled.

Next, a c-plane sapphire substrate with a thickness of 0.45 mm wasplaced on a susceptor disposed in the film forming chamber, and heatedsuch that the substrate temperature reached 450° C.

Next, with a 2.4-MHz ultrasonic transducer, ultrasonic vibration waspropagated through water to the precursors in the sprayers A, B, so thatthe precursors were atomized (mists were generated).

Next, a nitrogen gas was added to the sprayer B at a flow rate of 5L/min. The resulting mixture gas of the atomized precursor and thenitrogen gas was supplied to the reactor for 5 minutes, and a middlelayer having a film thickness of approximately 70 nm was formed.Immediately thereafter, the nitrogen-gas supply to the sprayer B wasstopped, and the mixture-gas supply to the reactor was stopped.

Next, a nitrogen gas was added to the sprayer A at a flow rate of 5L/min. The resulting mixture gas of the mist and the nitrogen gas wassupplied to the reactor for 30 minutes, and a crystal layer having afilm thickness of approximately 300 nm of formed. Immediatelythereafter, the nitrogen-gas supply to the sprayer A was stopped, andthe mixture-gas supply to the reactor was stopped.

Next, the substrate heating was stopped. The substrate was cooled toaround room temperature and taken out of the film forming chamber.

In the X-ray diffraction measurement, peak appeared at 2θ=40.3°. Thisconfirmed that the prepared crystal layer of the laminate was α-phaseGa₂O₃.

Then, the prepared laminate was analyzed with a transmission electronmicroscope (TEM). Moreover, the silicon concentration in the laminatewas analyzed by energy-dispersive X-ray spectroscopy (EDX).

Comparative Example

A crystal layer was formed using the apparatus used in Example fromwhich the sprayer B and the silicone resin pipe had been removed.

First, the sprayer A was filled with a precursor equivalent to that usedin Example.

Next, a c-plane sapphire substrate with a thickness of 0.45 mm wasplaced on a susceptor disposed in the film forming chamber, and heatedsuch that the substrate temperature reached 450° C.

Next, with the 2.4-MHz ultrasonic transducer, ultrasonic vibration waspropagated through water to the precursor in the sprayer A, so that theprecursor was atomized (mist was generated).

Next, a nitrogen gas was added to the sprayer A at a flow rate of 5L/min. The resulting mixture gas of the mist and the nitrogen gas wassupplied to the reactor for 35 minutes, and a crystal layer was formed.Immediately thereafter, the nitrogen-gas supply to the sprayer A wasstopped, and the mixture-gas supply to the film forming chamber wasstopped.

Next, the substrate heating was stopped. The substrate was cooled toaround room temperature and taken out of the film forming chamber.

In the X-ray diffraction measurement, peak appeared at 20=40.3°. Thisconfirmed that the prepared crystal layer was α-phase Ga₂O₃.

Then, the prepared laminate was analyzed with a transmission electronmicroscope (TEM). Moreover, the silicon concentration in the laminatewas analyzed by EDX.

FIG. 9 and FIG. 10 show sectional TEM images of the laminates preparedin Example and Comparative Example, respectively. It can be seen that,in Example shown in FIG. 9 , the middle layer including columnar crystalregions was formed on the substrate. In the middle layer of thesectional TEM image, dark columnar portions are crystal regions. Imageanalysis was conducted on the same sample at multiple positions in theTEM image. As a result, the proportion of the crystal region in middlelayer was approximately 9.5%. It should be noted that no middle layerwas formed in Comparative Example shown in FIG. 10 .

Moreover, looking at the crystal layer in Example, it can be seen thatthe crystal layer on the middle layer had large grown crystal grains(whitish portions) utilizing as seed crystals the columnar crystals inthe middle phase. In comparison with the crystal layer of ComparativeExample shown in FIG. 10 , it can be seen that crystal defects wereconsiderably reduced, which were observed as black contrast extending inthe film growth direction.

FIG. 11 shows concentration distributions of silicon atoms in the fieldsof view of FIGS. 9, 10 . In Example (FIG. 9 ), in a region approximately70 nm from the substrate surface where the middle layer abutted,approximately 2.0 at % of silicon was detected at its maximum. On theother hand, the silicon concentrations in the crystal layers of Exampleand Comparative Example were at the noise level and undetectable.

The above results have revealed that a low-defect, high-quality crystallayer (crystal film) is obtained according to the present invention incomparison with the conventional technique.

Moreover, in the present invention, since the middle layer and thecrystal layer are formed using one apparatus, the low-defect,high-quality crystal layer (crystal film) can be obtained at quite lowcost and high productivity in comparison with film formation using anadditional apparatus for forming an ELO growth mask and film formationrequiring photolithography process as in the conventional ELO method.Furthermore, since the present invention does not require an additionalapparatus as described above, there is a low possibility that thecrystal substrate is contaminated.

It should be noted that the present invention is not limited to theabove-described embodiments. The embodiments are just examples, and anyexamples that substantially have the same feature and demonstrate thesame functions and effects as those in the technical concept disclosedin claims of the present invention are included in the technical scopeof the present invention.

1. A mist-CVD apparatus comprising: a first atomizer for atomizing afirst metal oxide precursor and generating a first mist of the firstmetal oxide precursor; a second atomizer for atomizing a second metaloxide precursor and generating a second mist of the second metal oxideprecursor; a carrier-gas supplier for supplying a carrier gas to conveythe first and second mists; a film-forming unit for forming a film on asubstrate by subjecting the first and second mists to a thermalreaction; and a first conveyance pipe through which the first mist andthe carrier gas are conveyed to the film forming chamber, a secondconveyance pipe through which the second mist and the carrier gas areconveyed to the film forming chamber, wherein the carrier-gas supplieris capable of adding silicon to the carrier gas, and/or the firstatomizer is capable of adding silicon to the first metal oxideprecursor, and/or the first conveyance pipe is capable of adding siliconto the first mist and the carrier gas.
 2. The mist-CVD apparatusaccording to claim 1, wherein at least part of the first conveyance pipethrough which the first mist and carrier gas are conveyed to thefilm-forming unit comprises a pipe, which is mainly composed of asilicone resin.
 3. A film forming method by mist-CVD method comprising:forming a mixture gas containing at least an atomized metal oxideprecursor, a carrier gas, and silicon; supplying the mixture gas onto aheated crystal substrate; and forming a layer comprising a mixture of anamorphous region in an amorphous phase and a crystal region in a crystalphase having a corundum structure mainly made of a metal oxide.
 4. Thefilm forming method according to claim 3, wherein the mixture gas isformed by adding silicon while the carrier gas and the atomized metaloxide precursor are being conveyed toward the crystal substrate.
 5. Thefilm forming method according to claim 4, wherein the silicon is addedby conveying the atomized metal oxide precursor and the carrier gasthrough a silicone resin tube.
 6. The film forming method according toclaim 5, wherein the layer comprising the mixture of the amorphousregion in the amorphous phase and the crystal region in the crystalphase is formed by adjusting an amount of the mixture gas supplied tothe heated crystal substrate.